Many ceramic materials play an important role in modern technology. One example, ferrite materials are used in such diverse applications as inductor cores for filters, high-frequency transformer cores, magnetic memory cores, magnetic recording tapes, magnetic recording heads, and magnets. As another example, .beta.-alumina is used as a solid ionic conductor, and barium titanates ceramics are used extensively as capacitors.
Considerable effort has been expended in developing techniques to prepare various classes of ceramic materials with desired properties. A method that has applicability to a wide range of ceramic materials uses solid state reactions, at high temperatures, between the oxides or carbonates of the cation constituents in the ceramics. The method typically mixes very fine particles of the cation oxides or carbonates, and then calcines, mills, granulates, presses and sinters the composition. The milling step is frequently done in a slurry of the ceramic material and is often followed by spray drying of the ceramic slurry.
While there are many variations in the processing sequence just described, most variations, however, include both calcining and ball milling steps. The ball milling step is generally required because of a desire to thoroughly mix the constituents of the ceramic material to obtain a homogeneous ceramic composition. The desire for a homogeneous composition is even greater for those compositions in which the essential properties depend upon additives or dopants which are added to the ceramic in relatively small amounts. Such ceramics will be referred to as lightly doped and the additives will generally constitute 10 weight percent or less of the total ceramic. A homogeneous composition is desired for such material because aggregations of additives, if present in relatively small amount, may adversely alter material properties.
Such lightly doped ceramics are of commercial interest today. For example, Al.sub.2 O.sub.3, lightly doped with magnesium, typically in an amount of approximately 0.3 percent, is useful as an arc chamber in high-temperature sodium-vapor lamps. In this ceramic, the magnesium additive promotes densification in alumina. As a further example, metal oxides having small amounts of selected additives have been shown to possess non-ohmic properties that make them useful in varistor devices. Zinc oxide with additives such as bismuth, cobalt, manganese, chromium, antimony; and titanium dioxide with additives selected from the elements of Groups IIA and VB of the Periodic Table are of particular interest for varistor applications.
While the conventional method does produce ceramics having desirable properties, it is undesirable because of its time consuming, typically several days, nature. The conventional method, which requires calcining is not only time consuming, but the calcining step may introduce contaminants from either the boat or oven lining which adversely alter material properties.